Whole-body absorption of 2450-MHz radiation was measured in rats that ranged from 6 to 440 grams and in mice that ranged from 30 to 50 grams. Simultaneous exposure of groups of animals in varying numbers and in various configurations were made under free-field conditions in an electrically anechoic chamber. Measurements of whole-body absorption were made with twin-well calorimeters. The results indicate that, for animals within a given narrow range of body mass, individual rates of absorption may, at the extreme, vary by a factor of seven. One implication of this variability is that studies of dose-response relations may be confounded unless power densities of incident radiation are sufficiently different to provide nonoverlapping dose rates. Comparisons of measured doses to predicted values as derived from a spherical shell model show considerable variations of observed from predicted values. Specific relationships of the SAR to the position and mass of the animals are presented.